How to enter the competition? You just have to play the wave during the event's period and try the get the best score. Even if you ride only one wave you'll appear in the ranking.
How is the ranking calculated? For each day of the period, your rank will be used to give you points and establish the ranking. If you finish first you get 0 point, if you're second you get 1 point, third you get 2 points and so on. If you're last you get number of players points minus 1. If you don't play you get number of players points.
Quick advice: The best 4 days are added to make the final result.